02-17-2012, 09:31 PM
Palm/Maui- Thanks for your input. I will give you my opinion on some of your comments.
1. “GLJ’s Arun comment at Ant I believe was aimed at its potential productivity % and not the size of that formation”
Yes the analogy of Antelope being another Arun has been very much over played. About all they have in common is they are both thick limestone reefs. Here is some information from one of my old posts:
Some history of Arun Field
Discovery well had 1100 feet of reefal limestone.(Antelope-1 had around 2,300 ft.)
Production began in 1977.
Average reservoir depth 10,050 feet.
Original reservoir pressure was 7100 psi in 1977.
After producing for 12 years the reservoir pressure was still 4330 psi in 1989.
The original reserve estimate in 1975 was 16.1 TCF.
Field size 23,240 acres. (2+ times the size of Antelope but less that half as thick. The reason the recoverable reserves are some 2x higher than Antelope is the much higher original reservoir pressure. The original reservoir pressure at Antelope is about 3660 psi depending of the depth the pressure is measured. So original pressure at Arun was almost twice the pressure at Antelope.)
Arun Field has now been producing for 34 years.
The largest number I see here for possible recoverable gas from Arun field is 20 TCF.
2. “how large would Triceratops need to be horizontally in km to start approaching even half the volume that Arun is?”
It is already large enough to contain more than half the volume of Arun if we find a significant amount of reefal reservoir rock in all of the high spots that look like possible reefs. Arun field was 23,240 acres. If Triceratops is 62 square miles that is 39,680 acres. The discovery well at Arun had 1100 feet of reefal limestone. I think Triceratops may have a gas column height in excess of 2,000 feet. Arun was more than twice as deep as Triceratops so the higher pressure at Arun means an equal pore space would contain about 2.4 times as much gas at Arun as compared to Triceratops and about 1.9 times as much as Antelope. The original reservoir pressure at Arun was about 7,100 psi. I expect the pressure at Triceratops to be about 3,000 psi.
3. “With this formation being less thick than Arun it seems it would be a great candidate for directional drilling.”
Actually I think the formation at Triceratops will be thicker than Arun. As far as directional drilling is concerned, I believe we will see multiple wells from individual location pads. If we find high porosity/permeability reefal limestone the reservoir can probably be depleted without drilling the wells too close together. I believe the main reason for using directional drilling will be to save money on rig moves (helicopter), location preparation, roads and pipeline rights-of-way. The Triceratops Field area is very rough terrain (much worse than Antelope) and it will be expensive to build roads, locations and pipeline rights-of-way in this area. If we minimize the number of locations we will minimize the other requirements.
The more shallow depth of the Triceratops Field limits the “reach” of directional wells (as compared to Arun for example) unless they do go horizontal. I think they could develop an area up to 1 km in all directions from a single location. So location pads could be 3+ km apart without any problems. I think we will also likely see directional wells at Antelope.
4. “could any of the string of locations and/or high spots support multiple wells (perhaps in diff directions) from a single pad?”
I believe this is answered in No. 3 above.
5. Regarding the need for horizontal wells, I think this is misunderstood in some quarters.
Horizontal wells were originally used to increase the number of feet of “net” pay by drilling horizontally in a relatively thin pay zone or to get a much greater amount of pay zone open in low permeability reservoirs (shales, shaley sands, or shaley limestones). This allows the use of multi-zone frac treatments which allows for commercial production whereas a vertical well would produce uneconomical or less economical results.
In the case of very thick high permeability zones such as Antelope (and I hope Triceratops) horizontal wells are not required for the above described reasons.
At Antelope-2 I think IOC used the horizontal drilling for the following reasons:
a. They thought they had an oil column which they wanted to prove and test.
b. They wanted to show that the reservoir rock extended some distance from the vertical well.
c. They needed to pin down the gas/water contact if no oil zone was present.
Due to the higher expense and difficulty of horizontal drilling in this over pressured high permeability (loss of circulation) formation I would expect IOC to utilize directional drilling rather than horizontal drilling. Even with conventional vertical and directional drilling I think they will continue to have to use the pressure managed or mud cap drilling technique due to loss of circulation.